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Births, deaths and momentum

Population Growth Explained Simply

Population growth sounds simple, but it is driven by several moving parts: births, deaths, migration, life expectancy, fertility and age structure.

Population grows when births plus immigration exceed deaths plus emigration.

At the global level migration cancels out, so the difference between births and deaths is central.

The basic equation

For a country, population change equals births minus deaths plus immigration minus emigration. For the whole world, there is no external migration, so global growth depends on births and deaths.

Why growth can continue while fertility falls

If a population has many young adults, many children can be born even when the average number of children per woman is falling. This is called population momentum.

The demographic transition

Many societies first experience falling death rates while birth rates remain high. Population then grows quickly. Later, birth rates fall as education, urbanization, health care and economic structures change. Growth slows and populations may age.

Related topics

How many people are born per day?How many people die per day?Demography explained

FAQ

Can population grow if fertility is falling?

Yes, if there are many people of childbearing age and mortality is low.

Is population growth the same everywhere?

No. Countries can grow, shrink, age or remain stable depending on births, deaths and migration.